Abstract:Time-resolved electron diffraction with atomic-scale spatial and temporal resolution was used to unravel the transformation pathway in the photoinduced structural phase transition of vanadium dioxide. Results from bulk crystals and single-crystalline thin-films reveal a common, stepwise mechanism: First, there is a femtosecond V−V bond dilation within 300 fs, second, an intracell adjustment in picoseconds and, third, a nanoscale shear motion within tens of picoseconds. Experiments at different ambient temperat… Show more
“…S5 ). Such multi-stage diffraction evolution is qualitatively consistent with the earlier UED studies 34 , 35 , 37 , 38 , although we will point out the distinct differences found here in a strained ultrathin specimen. Figure 4 Time-resolved diffraction changes and intracell motions of vanadium ions.…”
Section: Resultssupporting
confidence: 91%
“…3c , blue). Moreover, no long-term lattice expansion is seen at F ex < 5 mJ/cm 2 , whose excitation threshold behavior coincides with those reported in previous time-resolved studies 26 – 29 , 34 , 35 , 37 , 38 . These results, as well as the long-time dynamics discussed below, suggest that the photoinjected energy is redistributed among multiple components of the structural dynamics, including the overshooting and then reduced amount of strain release as well as the intracell ionic motions along different directions (see below).…”
Section: Resultssupporting
confidence: 90%
“…These results, as well as the long-time dynamics discussed below, suggest that the photoinjected energy is redistributed among multiple components of the structural dynamics, including the overshooting and then reduced amount of strain release as well as the intracell ionic motions along different directions (see below). We argue that this may be one of the reasons why an energy density much higher than the enthalpy change (translated to be ~5 mJ/cm 2 with 1 to 2-eV photons 35 ) is needed to cause the complete transition from M 1 to R by photoexcitation, even though an onset threshold close to the thermodynamic requirement has often been seen for ultrafast initiation of the structural phase change in bulk and thicker-film specimens 28 , 35 .…”
Section: Resultsmentioning
confidence: 95%
“…From the structural point of view, it is critical to understand, at the atomic level, phase-transition dynamics of a correlated material during the entire transformation process. To date, the reaction path for photoinduced structural phase transition of VO 2 has been visualized using time-resolved diffraction methods on bulk 34 , 35 and strain-free specimens 35 – 38 . The general picture is that on the ultrashort time scale of few hundred femtoseconds (fs) or less, dilation of V‒V dimers in the initial low-temperature monoclinic structure ( M 1 ) plays a critical role in the beginning stage of the phase change 11 , 34 , 37 , 39 .…”
The complex phase transitions of vanadium dioxide (VO2) have drawn continual attention for more than five decades. Dynamically, ultrafast electron diffraction (UED) with atomic-scale spatiotemporal resolution has been employed to study the reaction pathway in the photoinduced transition of VO2, using bulk and strain-free specimens. Here, we report the UED results from 10-nm-thick crystalline VO2 supported on Al2O3(0001) and examine the influence of surface stress on the photoinduced structural transformation. An ultrafast release of the compressive strain along the surface-normal direction is observed at early times following the photoexcitation, accompanied by faster motions of vanadium dimers that are more complex than simple dilation or bond tilting. Diffraction simulations indicate that the reaction intermediate involved on picosecond times may not be a single state, which implies non-concerted atomic motions on a multidimensional energy landscape. At longer times, a laser fluence multiple times higher than the thermodynamic enthalpy threshold is required for complete conversion from the initial monoclinic structure to the tetragonal lattice. For certain crystalline domains, the structural transformation is not seen even on nanosecond times following an intense photoexcitation. These results signify a time-dependent energy distribution among various degrees of freedom and reveal the nature of and the impact of strain on the photoinduced transition of VO2.
“…S5 ). Such multi-stage diffraction evolution is qualitatively consistent with the earlier UED studies 34 , 35 , 37 , 38 , although we will point out the distinct differences found here in a strained ultrathin specimen. Figure 4 Time-resolved diffraction changes and intracell motions of vanadium ions.…”
Section: Resultssupporting
confidence: 91%
“…3c , blue). Moreover, no long-term lattice expansion is seen at F ex < 5 mJ/cm 2 , whose excitation threshold behavior coincides with those reported in previous time-resolved studies 26 – 29 , 34 , 35 , 37 , 38 . These results, as well as the long-time dynamics discussed below, suggest that the photoinjected energy is redistributed among multiple components of the structural dynamics, including the overshooting and then reduced amount of strain release as well as the intracell ionic motions along different directions (see below).…”
Section: Resultssupporting
confidence: 90%
“…These results, as well as the long-time dynamics discussed below, suggest that the photoinjected energy is redistributed among multiple components of the structural dynamics, including the overshooting and then reduced amount of strain release as well as the intracell ionic motions along different directions (see below). We argue that this may be one of the reasons why an energy density much higher than the enthalpy change (translated to be ~5 mJ/cm 2 with 1 to 2-eV photons 35 ) is needed to cause the complete transition from M 1 to R by photoexcitation, even though an onset threshold close to the thermodynamic requirement has often been seen for ultrafast initiation of the structural phase change in bulk and thicker-film specimens 28 , 35 .…”
Section: Resultsmentioning
confidence: 95%
“…From the structural point of view, it is critical to understand, at the atomic level, phase-transition dynamics of a correlated material during the entire transformation process. To date, the reaction path for photoinduced structural phase transition of VO 2 has been visualized using time-resolved diffraction methods on bulk 34 , 35 and strain-free specimens 35 – 38 . The general picture is that on the ultrashort time scale of few hundred femtoseconds (fs) or less, dilation of V‒V dimers in the initial low-temperature monoclinic structure ( M 1 ) plays a critical role in the beginning stage of the phase change 11 , 34 , 37 , 39 .…”
The complex phase transitions of vanadium dioxide (VO2) have drawn continual attention for more than five decades. Dynamically, ultrafast electron diffraction (UED) with atomic-scale spatiotemporal resolution has been employed to study the reaction pathway in the photoinduced transition of VO2, using bulk and strain-free specimens. Here, we report the UED results from 10-nm-thick crystalline VO2 supported on Al2O3(0001) and examine the influence of surface stress on the photoinduced structural transformation. An ultrafast release of the compressive strain along the surface-normal direction is observed at early times following the photoexcitation, accompanied by faster motions of vanadium dimers that are more complex than simple dilation or bond tilting. Diffraction simulations indicate that the reaction intermediate involved on picosecond times may not be a single state, which implies non-concerted atomic motions on a multidimensional energy landscape. At longer times, a laser fluence multiple times higher than the thermodynamic enthalpy threshold is required for complete conversion from the initial monoclinic structure to the tetragonal lattice. For certain crystalline domains, the structural transformation is not seen even on nanosecond times following an intense photoexcitation. These results signify a time-dependent energy distribution among various degrees of freedom and reveal the nature of and the impact of strain on the photoinduced transition of VO2.
“…As shown in numerous works, 22,[38][39][40][41][42][43][44][45] the photoinduced IMT in VO 2 depends on incident light intensity, wavelength, and also on film crystallinity, structural defects, and internal strain. Recent studies indicate that the ultrafast structural phase transition (SPT) is likely to be triggered by a screening of electron correlations on the sub-picosecond scale.…”
Distinct contribution of acoustic and optical phonons in light-induced lattice transformation was resolved at different time scales by monitoring the insulator-to-metal phase transition in epitaxial and non-epitaxial VO2 films. Applying the ultrafast angle-resolved light scattering technique we demonstrate a significant influence of internal misfit strain in epitaxial films on sub-picosecond phase transition dynamics. This technique also allows observing a contribution of structural defects in the evolution of the transient state. The ultrafast structural phase transition dynamics is discussed in terms of the Ginzburg-Landau formalism. Using a set of experimental data we reconstruct the thermodynamic potential of photoexcited VO2 and provide a phenomenological model of ultrafast light-induced structural phase transition.
The fields of brain‐inspired computing, robotics, and, more broadly, artificial intelligence (AI) seek to implement knowledge gleaned from the natural world into human‐designed electronics and machines. In this review, the opportunities presented by complex oxides, a class of electronic ceramic materials whose properties can be elegantly tuned by doping, electron interactions, and a variety of external stimuli near room temperature, are discussed. The review begins with a discussion of natural intelligence at the elementary level in the nervous system, followed by collective intelligence and learning at the animal colony level mediated by social interactions. An important aspect highlighted is the vast spatial and temporal scales involved in learning and memory. The focus then turns to collective phenomena, such as metal‐to‐insulator transitions (MITs), ferroelectricity, and related examples, to highlight recent demonstrations of artificial neurons, synapses, and circuits and their learning. First‐principles theoretical treatments of the electronic structure, and in situ synchrotron spectroscopy of operating devices are then discussed. The implementation of the experimental characteristics into neural networks and algorithm design is then revewed. Finally, outstanding materials challenges that require a microscopic understanding of the physical mechanisms, which will be essential for advancing the frontiers of neuromorphic computing, are highlighted.
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